Condensation oscillations in the peptidization of phenylglycine

The remarkable phenomenon of spontaneous in vitro oscillatory chiral conversion of selected profens, amino acids, and hydroxy acids has been the focus of our attention for some time. We first observed oscillatory chiral conversion in selected profens: S(+)-ibuprofen, S(+)-naproxen, S,R(±)-2-phenylpropionic acid, S(+)-flurbiprofen, R(-)-flurbiprofen, and S,R(±)-ketoprofen [1-3]. Later we described an analogous behavior in several amino acids: L-alanine, L-α-phenylalanine, and L-tyrosine [4-6]. We have characterized similar phenomena in selected hydroxy acids [7,8] (L-lactic acid, R-α-hydroxybutyric acid, S-α-hydroxybutyric acid, R-mandelic acid, and S-mandelic acid) as well. Most of these oscillatory chiral conversions take place with the acid samples dissolved in 70% aqueous ethanol.The general scheme of these chiral conversions can be simply summarized asIf we consider chiral conversion of selected carboxylic acids in aqueous solution in greater detail, then eq. (1) can be written as [9]:In anhydrous media and in the presence of trace amounts of water, the probable mechanism of chiral conversion is given by eq. (1b) [10]:where X = -NH2, -OH, -Ar, etc., and Y = -R, etc.We have suggested two simple models of oscillatory chiral conversion [11]. The simpler model [5,12] is based on autocatalytic dimerization. Oscillations appear in this model in both enantiomer subsystems. Racemization is a linear reversible reaction coupling the two oscillating subsystems. This model is based on the oscillatory Templator model of Peacock-López and collaborators [13,14].In the second approach [15], oscillations appear as the result of coupling between enantiomers. This model appears particularly relevant for systems involving condensation or peptidization in which the condensate (dipeptide) SR has different chemical properties than RS, e.g., where only the residue on the left undergoes epimerization.In subsequent papers [16-18], we demonstrated via the biuret test [16] and 13C NMR spec